1. Field of the Invention
This invention generally relates to apparatus for closing containers and more particularly to the liquid-tight ultrasonic sealing of bottom closures of cartons composed of foldable material double coated with thermoplastic, examples of which are polyethylene double coated paperboard containers used for the packaging of milk and other dairy products.
2. Description of the Prior Art
Over the past three decades the packaging industry has witnessed revolutionizing changes in the art. In the milk packaging industry, the industry in which the present invention is likely to find its greatest application, containers have evolved from returnable glass bottles--to wax coated paperboard--to paperboard containers coated with thermoplastic. Carbon blanks for the latter type cartons are customarily cut and scored for folding and joined along appropriate edges with a liquid tight side seam by paper companies to form a collapsed tube of material whereafter the carton blanks are supplied to milk packagers in a collapsed or flat form. Final packaging generally requires expanding the carton blank to its tubular shape, forming a liquid-tight closure at the carbon bottom, filling the carton, and thereafter closing the carton top with a liquid tight seal such as, for example, the conventional gable top pour spout closure.
The prior art which is most germane to the present invention is that which relates to apparatus for forming carton bottom closures with liquid-tight seals. In the case of thermoplastic double coated paperboard containers, the panels and tabs of the bottom closures, being defined by the carton blank profile and scoring patterns, are conventionally subjected to gas or electric radiant heat or superheated blown air which activates the thermoplastic double coating. Thereafter the carton bottom closure is folded into, and held under pressure in, its closed position until the activated thermoplastic is cooled to its solid state forming a liquid-tight seal between adjoining carton bottom panels and tabs. Exemplary conventional folding and sealing techniques for top and bottom closures are disclosed in U.S. Pat. No. 3,120,335 to Engleston et al. and U.S. Pat. No. 3,120,333 to Seiple.
Prior art gas and electric radiant heat and superheated blown air sealing techniques have long been recognized as objectionable because, among other reasons, it is difficult with such techniques to maintain consistency of heat distribution and to control the activation of the thermoplastic in the sealing process. In addition, the packaging area of the plant is often overheated during the sealing process to an uncomfortable level and such heat poses continuous fire hazards.
To eliminate the above-noted shortcomings ultrasonic sealing techniques have been developed with some success. For example, there have been breakthroughs in the art which now make it possible to ultrasonically seal the carton top closure thereby avoiding the need for gas or electric radiant heat and superheated blown air in all packaging steps after carton bottom closure. An example of the ultrasonic top closure technique is disclosed in U.S. Pat. No. 3,486,731 to Obeda. There have been some developments and experimentations with respect to the use of ultrasonic techniques for joining laminates of varying thicknesses by the use of contour surfaces on ultrasonic sealing apparatus. One such development is described in German Pat. application DAS No. 2165620. However, these techniques have heretofore been unsatisfactory for use in carton bottom closure sealing because of the difficulties, including uncontrolled "burn through", presented by the sealing of a closure on a tubular container, particularly when the closure comprises a multiplicity of laminated surfaces of varying thicknesses and complex shapes and sections of the closure are not included in the ultrasonic seal pattern.
Thus, it would be a desirable advance in the art to provide apparatus to overcome heretofore encountered difficulties, including uncontrolled burn through, while eliminating completely the need for gas or electric radiant heat and superheated blown air in sealing carton bottom closures. Providing such apparatus for use in closing the bottom closures of thermoplastic double coated paperboard containers such as polyethylene double coated paperboard cartons eliminates altogether the need for gas or electric radiant heat and superheated blown air in the packaging of liquids as, for example, in the packaging of milk and other dairy products.